neutral point (V
3N
) will be close to zero in case of a stator ground-fault close to the neutral. This
fact alone can be used as an indication of stator ground-fault. To enable better sensitivity and
stability also measurement of the generator's 3
rd
harmonic voltage V
3T
is also used. In addition to
the decrease of V
3N
the generator voltage V
3T
will increase under the stator ground-fault close to
the generator neutral point. Therefore the 3
rd
harmonic voltage V
3T
, (which is a zero sequence
voltage) is used by the protection. In the 3
rd
harmonic voltage differential protection algorithm
equation 198 is used:
EQUATION2359-ANSI V1 EN-US (Equation 198)
V
3N
, and V
3T
are third harmonic phasors with real and imaginary parts. The factor
Beta
must be set
not to risk operation under non-faulted conditions.
The voltage V
3N
is measured via a voltage transformer between the generator neutral point and
ground. The voltage V
3T
can be measured in different ways. The setting
TVoltType
defines how the
protection function is fed from voltage transformers at the high voltage side of the generator. If
V
3T
is lower than the set level
VT3BlkLevel
, STEFPHIZ (59THD) function is blocked. The choices of
TVoltType
are:
NoVoltage
: There is no voltage measured from the generator terminal side. This can be the case
when there are only phase-to-phase voltage transformers available at the generator terminal side.
In this case the protection will operate as a simple neutral point 3
rd
harmonic undervoltage
protection, which must be blocked externally during generator start-up and shut-down.
ResidualVoltage
: The function is fed from a broken delta connection of the phase to ground
connected voltage transformers at the generator terminal side, V
3T
=(1/3)*V_Broken_Delta.
AllThreePhases
: The function is fed from the three phase to ground connected voltage
transformers at the generator terminal side. The 3
rd
harmonic voltage V
3T
is calculated in the IED,
V
3T
=(1/3)*(V3A+V3B+V3C).
PhaseA
,
PhaseB
, or
PhaseC
: The function is fed from one phase voltage transformer only. The 3
rd
harmonic zero sequence voltage is assumed to be equal to any of the phase voltages, as the third
harmonic voltage is of zero sequence type, V
3T
=V3Lx.
A simplified block diagram describing the stator ground fault protection function shown in
figure
356.
1MRK 502 066-UUS B Section 9
Voltage protection
657
Technical manual
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